This invention relates to the fabrication of ohmic contacts to Group III(a)-V(a) compound semiconductors.
The problems associated with making ohmic contacts to Group III(a)-V(a) semiconductor devices, such as GaAs field effect transistors (FETs), are many and varied. As pointed out by W. T. Anderson et al (J. Appl. Phys., Vol. 49, No. 5, pp. 2998-3000, May 1978), ohmic contacts of commercially available GaAs FETs typically show island formation on the surface, deteriorate on aging, yield reproducibly low resistances only with difficulty, require critically short alloying times, and exhibit cracking and enhanced diffusion as a result of strain.
The critically short alloying times alluded to by Anderson et al are related to the common practice of evaporating eutectic alloys, such as Au-Ge, on n-type GaAs. Because the alloy as deposited forms a Schottky barrier, the contact is heated above the eutectic temperature (about 360.degree. C. for Au-Ge) for as short a time as possible to melt the alloy and transform the rectifying contact into an ohmic one. This alloying procedure is also commonly used in the fabrication of ohmic contacts (e.g., using Sn-Pd-Au alloys) to n-type layers of GaAs semiconductor lasers. In contrast, the fabrication of ohmic contacts to p-type GaAs is much simpler and requires no alloying because most metals do not form Schottky barriers on p-type GaAs or other p-type Group III(a)-V(a) compounds, but rather form ohmic contacts directly.